Conventionally, the drilling of a well into the earth by rotary drilling techniques, involves the circulation of a drilling fluid from the surface of the earth down a drill string having a drill bit on the lower end thereof and through ports provided in the drill bit to the well bottom and thence back to the surface through the annulus formed about the drill string. The drilling fluid serves to cool the drill bit, to transport drill cuttings to the surface, and to stabilize the wellbore.
Wells such as oil, gas or water injection wells are frequently drilled or completed using oil-based wellbore fluids, that is, fluids having a continuous oil phase. Such fluids are often referred to as oil-based muds. Oil-based muds usually contain a brine phase dispersed in the continuous oil phase, water-in-oil emulsifiers, dispersed solid particles such as barite and calcium carbonate, and oil-wetting agents to maintain such particles in an oil-wet condition. They also frequently contain dissolved or colloidally-dispersed polymers or resins such as gilsonite, blown asphalt, vinyl toluene/alkyl acrylate copolymers or amine-treated lignite. These dissolved or colloidally dispersed materials act to reduce the filtration rate of the wellbore fluid. As well as finding application as drilling fluids, such fluids can also be used as completion or workover fluids.
When drilling a permeable rock formation such as a hydrocarbon producing formation or an interval intended for water injection (e.g. seawater injection), the hydrostatic pressure in the well is maintained higher than the natural pressure of fluids inside the permeable rock in order to prevent a flow of formation fluids into the well. This differential pressure causes filtration of the drilling fluid such that a substantially all-oil filtrate is forced into the permeable rock. The dispersed particles such as oil-wet barite are generally too large to enter the pores of the formation, and so they are deposited on the wellbore wall as a filter-cake. The oil-wet particles pack together in the filter-cake under the applied differential pressure, and the strongly emulsified brine emulsion droplets become trapped in the pores between the solid particles. Because the brine is a non-wetting phase the droplets cannot readily be forced through the pore throats, thus plugging the pore throats and substantially reducing the permeability of the filter-cake. The filter-cake permeability is yet further reduced by the accumulation of dissolved or colloidally-dispersed polymers or resins sealing the pore passages.
During drilling, the very low permeability filter-cake fulfils the valuable function of limiting losses of filtrate to the formation, and avoids problems such as differential sticking of the drillstring. Unfortunately the low permeability filter-cake can later form a barrier to the production of hydrocarbons, or to the injection of seawater, especially if it becomes trapped between the formation and the expanded screens or gravel packs that are commonly used to limit sand invasion into the wellbore. The problem is particularly acute in the case of seawater injection wells for two reasons. Firstly, the injected water pressure forces the filter-cake onto the rock surface, which can consolidate the seal. Secondly, oil-based mud filter-cakes are persistently oil-wet, which makes the penetration of seawater into and through the filter-cake during subsequent operations very difficult.
Accordingly the industry has employed filter-cake treatment fluids pumped down the well in attempts to reverse the wettability and break the emulsion in the filter-cake. Such “breaker” fluids typically contain surfactants, co-solvents, micro-emulsions and acids aimed at opening up the permeability of the filter-cake. U.S. Pat. No. 6,790,811 discloses one method of providing breakable filter-cakes where the emulsifiers are able to be protonated under acidic conditions. This is designed to make the filter-cake susceptible to acidic treatment fluids pumped into the well: acids have the effect of reversing the emulsion and water-wetting the solids in the filter-cake, rendering it permeable.
Unfortunately such treatments are frequently inefficient. The very low permeability of oil-based mud filter-cakes impedes the penetration of the breaker fluid. Often long “soak times” are required. Another problem is that if a portion of the filter-cake is quickly broken, the breaker fluid may then flow into the rock formation leaving the rest of the filter-cake untreated.
There is thus a need for oil-based muds which produce filter-cakes which are initially of low permeability, but which can be made to increase in permeability at a later stage, for example when the well is put onto production, or when water injection commences.
U.S. Pat. No. 5,057,234 discloses a wellbore fluid aimed at avoiding pollution or oil sheen upon discharge of wastes to the sea. It is a replacement fluid for an oil-based mud wherein the oil is replaced by a glycol that is soluble or dispersible in seawater having a salinity of about 3% and lower. The fluid is characterized by the absence of hydrocarbon, mineral, vegetable and animal oils. This wellbore fluid therefore contains two liquid phases: the glycol continuous phase and the emulsified brine.
WO1992/014798 discloses a drilling fluid aimed at avoiding pollution or oil sheen upon discharge of wastes to the sea. Here the oil in an invert emulsion drilling fluid is replaced by an organic fluid that is at least 40% soluble in water. It also is a wellbore fluid containing two liquid phases, the organic liquid and brine.
U.S. Pat. No. 5,990,050 discloses a drilling fluid aimed at avoiding pollution upon discharge of wastes (mud-covered cuttings) to the sea. The continuous liquid phase of the drilling fluid comprises or includes a preferentially-oil-soluble glycol ether which is miscible in oil and which is selected from propylene glycol n-butyl, ether (PnB); dipropylene glycol n-butyl ether (DPnB); tripropylene glycol n-butyl ether (TPnB); tetrapropylene glycol n-butyl ether; pentapropylene glycol n-butyl ether; hexapropylene glycol n-butyl ether; heptapropylene glycol n-butyl ether; and combinations thereof. The wellbore fluid of U.S. Pat. No. 5,990,050 therefore contains one homogeneous organic phase, and optionally an emulsified brine phase, i.e. one or two liquid phases.
GB 2 341 876 discloses a pumpable multiple phase emulsion, which consists of a treatment agent present in a first phase, the first phase being suspended in a second phase to form a first pumpable emulsion, and the first pumpable emulsion being dispersed in the third phase to form the pumpable three-phase composition. The resulting emulsion takes the form of either an oil phase-in-aqueous phase-in oil phase composition, or an aqueous phase-in-oil phase-in-aqueous phase composition.
WO 99/14285 discloses an electrically conductive wellbore fluid which comprises as its continuous phase a polar organic liquid. An oil may be present in admixture with the polar organic liquid.
WO 03/106587 also relates to electrically conductive wellbore fluids, and discloses a method of electrically logging subterranean wells using a conductive double emulsion fluid including an oil, an emulsifier capable of forming a microemulsion, an emulsifier capable of forming an invert emulsion, an electrolytic salt, and optionally a polar organic solvent.